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Date: Fri, 20 Oct 2017 09:37:30 +0200
From: Christoph Hellwig <hch@lst.de>
To: Jim Quinlan <jim2101024@gmail.com>
Subject: Re: [PATCH 5/9] PCI: host: brcmstb: add dma-ranges for inbound traffic
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Cc: Mark Rutland <mark.rutland@arm.com>, linux-mips@linux-mips.org,
 Florian Fainelli <f.fainelli@gmail.com>, devicetree@vger.kernel.org,
 linux-pci <linux-pci@vger.kernel.org>, Kevin Cernekee <cernekee@gmail.com>,
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 Rob Herring <robh+dt@kernel.org>,
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 Brian Norris <computersforpeace@gmail.com>,
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On Thu, Oct 19, 2017 at 06:47:45PM -0400, Jim Quinlan wrote:
> The only way to prevent this is to reserve a single page at the end of
> the first memory region of any pair that are adjacent in physical
> memory.  A hack, yes, but I don't see an easier way out of this.  Many
> if not most of our boards do not have adjacent regions and would not
> need this.

dma mappings can be much larger than a single page.  For the block
world take a look at __blk_segment_map_sg which does the merging
of contiguous pages into a single SG segment.  You'd have to override
BIOVEC_PHYS_MERGEABLE to prevent this from happening in your supported
architectures for the block layer.

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